1. Field of the Invention
The present invention relates to methods for manufacturing multilayer ceramic capacitors and other multilayer ceramic electronic components and to multilayer ceramic electronic components manufactured by these methods. More specifically, it relates to improvements for increasing the numbers of layers of ceramic layers and internal electrodes and for thinning these layers.
2. Description of the Related Art
Typical examples of multilayer ceramic electronic components in which the present invention is interested are multilayer ceramic capacitors.
With increasing demands on downsizing, increasing electrostatic capacity and reducing cost in multilayer ceramic capacitors, constitutive ceramic layers composed of a dielectric material have been thinned to a thickness of about 3 xcexcm. In addition, Cu, Ni and other base metals are used as conductive materials for internal conductor films, i.e., internal electrodes. Recently, multilayer ceramic capacitors each comprising further thinned ceramic layers about 1 xcexcm thick have been developed.
To increase the electrostatic capacity, the number of layers of internal electrodes for yielding electrostatic capacity has been increased. In an laminated body comprising ceramic layers laminated with the interposition of internal electrodes, portions carrying the internal electrodes have a thickness larger than that of portions carrying no internal electrodes. When the number of layers of the internal electrodes is increased as mentioned above, the portions carrying the internal electrodes have a thickness markedly larger than that of portions carrying no internal electrodes to thereby cause distortion of the resulting laminated body. To avoid this problem, individual internal electrodes must be further thinned.
Such internal electrodes are conventionally formed by subjecting a conductive paste comprising a dispersed metal powder to screen printing to thereby form a pattern of the conductive paste on ceramic green sheets to be the ceramic layers. If thin internal electrodes are formed by screen printing in this manner, electrode breaks frequently occur during co-firing with the ceramic, and the electrostatic capacity of the resulting multilayer ceramic capacitor is less than the designed level. The thickness of the internal electrodes cannot therefore sufficiently be reduced as long as they are formed by screen printing using a conductive paste.
Conductive pastes for use in screen printing are mixtures of a metal powder, a resin (a binder) and a solvent. Accordingly, the physical thickness of the screen-printed internal electrodes is about two to three times as large as that of the constitutive metal component. This also prevents mitigation of the distortion of the laminated body induced by the thickness of the internal electrodes.
As a possible solution to these problems, a metal film formed by a thin film forming method is used as the internal electrodes. When the metal film is used as the internal electrodes, its physical thickness becomes nearly equal to that of the metal powder, and the distortion of the laminated body induced by thickness of the internal electrodes can significantly be mitigated. In the aforementioned internal electrodes formed by screen printing using a conductive paste, the metal powder in the conductive paste may not be dispersed satisfactorily in the resulting internal electrode. In contrast, the internal electrodes comprising the metal film formed by the thin film forming method are free of this problem. Accordingly, this technique is effective to thin the internal electrodes also from this point of view.
The metal film formed by the thin film forming method is nearly free of pinholes and other defects even when its thickness is, for example, less than or equal to 1 xcexcm.
When a green laminated body comprising a plurality of ceramic green sheets and metal film laminated in alternate order is subjected to removal of a binder contained in the ceramic green sheet, i.e., to debinder (binder burnout), a gas is formed as a result of decomposition of the binder. The metal film is free of pinholes as described above and therefore prevents diffusion of the gas specifically in the lamination direction, thus preventing a sufficient debinder effect. In addition, the resulting multilayer ceramic capacitor tends to invite structural defects such as delamination at the interface between the metal film or the resulting internal electrodes and the ceramic green sheet or the resulting ceramic layers.
A possible solution to these problems is to reduce the amount of the binder (resin) in the ceramic green sheet. However, if the amount of the binder is reduced, the metal film internal electrode does not come into intimate contact with the ceramic green sheet properly when the metal film is brought into contact with the ceramic green sheet. Therefore, the amount of the binder in the ceramic green sheet must be increased in this technique as compared with the process in which the internal electrodes are prepared by screen printing using a conductive paste.
If the ceramic green sheet comprises an increased amount of the binder, the amount of the gas which is formed during the debinder process step as a result of decomposition of the binder increases. The gas formed in an increased amount should be diffused, but the metal film internal electrode prevents diffusion of the gas as described above, and the increased gas further frequently invites structural defects such as delamination at the interfaces between the internal electrodes and the ceramic layers.
In the debinder process step, the gas formed as a result of decomposition of the binder is generally emitted from pores formed as a result of burning of the binder in the ceramic green sheet, and the green laminated body itself shrinks during this process. Adhesion between the internal electrodes and the ceramic layers at the interfaces decreases as the decomposition of the binder proceed. The shrinkage of the green laminated body and the decreased adhesion may also cause structural defects such as delamination at the interfaces between the internal electrodes and the ceramic layers.
These structural defects occur more markedly with a decreasing thickness of the ceramic layer and with a decreasing grain size of the ceramic material powder in the ceramic green sheet. If the ceramic layer has a large thickness of, for example, more than 1.5 xcexcm, a ceramic material powder having a large grain size adapted to the thickness of the ceramic layer can be used. The amount of the binder essential for the ceramic green sheet can therefore be decreased to thereby decrease the amount of the gas formed as a result of decomposition of the binder. In addition, the green laminated body less shrinks during the debinder process step. Accordingly, structural defects caused by these factors, such as delamination at the interfaces between the internal electrodes and the ceramic layers, can be minimized.
Similar problems also occur in multilayer ceramic electronic components other than the multilayer ceramic capacitors.
Accordingly, it is an object of the present invention to provide a method for manufacturing a multilayer ceramic electronic component which can solve the above problems, as well as a multilayer ceramic electronic component manufactured by this method.
Specifically, the present invention provides, in a first aspect, a method for manufacturing a multilayer ceramic electronic component including the steps of preparing a ceramic green sheet including a ceramic material powder and a binder; preparing a metal film by a thin film forming method; forming a green laminated body by laminating a plurality of the ceramic green sheets and the metal films; removing the binder by subjecting the green laminated body to a heat treatment; and forming a sintered laminated body by firing the heat-treated green laminated body. To solve the above problems, the green laminated body is subjected to the heat treatment in a pressurized atmosphere at a gauge pressure exceeding 0.1 MPa in the step of removing the binder. By this configuration, abrupt evolution of a gas inside the green laminated body as a result of decomposition of the binder can be prevented.
Accordingly, structural defects caused by the decomposed gas, such as delamination at the interfaces between the metal film or the resulting internal electrodes and the ceramic green sheet or the resulting ceramic layers, can be minimized even if the electrode in the form of a film which prevents diffusion of the decomposed gas in the debinder process step is present in the green laminated body.
The gauge pressure in the step of removing the binder is preferably equal to or more than about 0.15 MPa. This configuration further effectively prevents production of the decomposed gas as a result of decomposition of the binder and evaporation of a plasticizer, if any, to enable the resulting multilayer ceramic electronic component to be resistant to such structural defects.
The ceramic material powder in the ceramic green sheet preferably has a grain size in a range from about 50 to 200 nm. This configuration enables the ceramic green sheet to be thinned and the resulting ceramic layers to be thinned to a thickness of, for example, less than or equal to about 1.5 xcexcm. However, the amount of the binder required for the ceramic green sheet increases with a decreasing grain size of the ceramic material powder. Accordingly, the present invention exhibits specifically marked advantages under these conditions.
A process selected from, for example, vapor deposition, sputtering, electroplating and chemical plating may be used as the thin film formation method to prepare the metal film.
In the method according to the present invention, it is preferred that the metal film is formed on a supporting member, and the metal film on the supporting member is transferred onto the ceramic green sheet to thereby yield the green laminated body.
The ceramic green sheet preferably further includes a plasticizer. Evaporation of the plasticizer can be prevented by the heat treatment in a pressurized atmosphere, and the ceramic green sheet can keep its plasticity even at temperatures at which the gas is produced as a result of decomposition of the binder. The resulting multilayer ceramic electronic component therefore becomes resistant to the structural defects mentioned above.
The present invention is also directed to a multilayer ceramic electronic component obtained by the manufacturing method. The multilayer ceramic electronic component includes ceramic layers formed from the ceramic green sheet, and metal films formed from the conductor.
In the multilayer ceramic electronic component, the ceramic layers each preferably have a thickness of less than or equal to about 1.5 xcexcm and the metal film each preferably have a thickness of less than or equal to about 0.8 xcexcm. This configuration is advantageous for increasing the number of layers in the resulting multilayer ceramic electronic component.
By applying the method for manufacturing a multilayer ceramic electronic component according to the present invention, for example, a multilayer ceramic capacitor can be manufactured. In this case, the plurality of the metal films are arranged so as to yield electrostatic capacity in the step of forming the green laminated body, and the method further includes the step of forming external electrodes on an outer surface of the sintered laminated body to thereby yield a multilayer ceramic capacitor.
The present invention is also directed to a multilayer ceramic capacitor obtained by the manufacturing method just mentioned above. The multilayer ceramic capacitor includes ceramic layers formed from the ceramic green sheet, internal electrodes formed from the conductor, and the outer electrodes.
In the multilayer ceramic capacitor, the ceramic layers each preferably have a thickness of less than or equal to about 1.5 xcexcm and the internal electrodes each preferably have a thickness of less than or equal to about 0.8 xcexcm. This configuration is advantageous for downsizing and achieving higher capacity of the multilayer ceramic capacitor.
The present invention, therefore, enables the multilayer ceramic electronic component to be thinned and to have an increased number of layers. When the present invention is applied to a multilayer ceramic capacitor, the invention advantageously enables downsizing and higher capacity of the multilayer ceramic capacitor.